Electron beam focusing structure



Feb. 16, 1960 R. E. MCCLURE ELECTRON BEAM FOCUSING STRUCTURE Filed July 28, 1955 5 Sheets-Shea?l 1 ATTORNEY wmf-@f Feb. 16, 1960 R. E. MccLuRE ELEcTRoN BEAM FocUsING STRUCTURE 3 Sheets-Sheet 2 Filed July 278. 1955 .4 C Q 0.0101040 Q Q Q o Q 0 o 0 o s Q 000000000 009000 T"ITRE mm RMA 2 Mm 3 Sheets-Sheet 3 2M ATTAQRNEY R. E- MCCLURE ELECTRON BEAM FOCUSING STRUCTURE WE1'. E

Feb. 16, 1960 Filed July 28. 1955 United safes Patent o 2,925,508 ELECTRN BEAM FocUsiNG STRUCTURE Robert E. McClure, Great Neck, N.Y., assigner to Sperry This invention pertains to magentic lield shaping apparatus and more particularly to a device for compensating for the reduction of magnetic field that occurs at gaps in solenoid magnets.

In electron beam tubes, such as-klystrons and traveling wave amplifiers, it is necessary to project a stream of electrons along the axis of the tube for a considerable distance. To coniine the beam of electrons to a given cross-section by preventing its radial dispersion, it is necessaryto employ an axial magnetic field. The axial magnetic iield tends to convert any radial motion of the electrons in the beam into a rotational motion `about the tube axis and thereby prevents spreading of the beam. This field is frequently provided by a solenoid external to the electron tube structure. axial magnetic eld and to prevent any fluctuations in the electron beam along its path,` it is desirable that the solenoid be uniform and coextensive with the path length traveled by the electron stream along the tube axis. In the construction of tubes of this type it is often necessary to couple energy into or out of the tube by means Aof` radially disposed transmission line or waveguide structures intermediate the ends of the tube. These coupling structures prevent the use of a uniform continuous solenoid surrounding the length of the electron beam path, but instead the solenoid must be constructed in segments, the segments terminating at the coupling devices and thereby defining a gap. The axial magnetic field in the vicinity of these gaps tends to fringe outwardly and become weakened. Such non-uniformity Vin the axial magnetic iield interferes with the beam focusing and the eiiicient operation of these electron tubes.

Itis therefore .an object of` this invention to provide means for overcoming the reductiondof magnetic eld that occurs at gaps in electron `beam focusing structures. `It is `a further `object of this invention to `provide, means for shaping axially symmetric magnetic fields in the vicinity of .solenoid gaps. i

-It is a further object of this invention to provide for a uniform axial magnetic eld despite gaps in netic structure. ,u u A v `It is a further object of this invention tominimize the fringing of magnetic elds near the ends of long solenoids. In this invention means is provided to compensate for the reduction of the axial magnetic iield `at gaps in the the magsolenoid. Compensation means is used in cooperation To provide a uniformwill become apparent from the specification taken in con= nection with the accompanying drawings, wherein, Fig. l is a cross-sectional view of a solenoid;

Fig. 2 is a cross-sectional view of a solenoid having l a sap;

Fig. 3 is a cross-sectional View of a saturated para'- magnetioshell having a gap;

u Fig. 4 is a cross-sectional view of the magnetic gap compensating structure of this invention;

Fig. 5 is a cross-sectional View of a klystron amplifier tube employing the compensating structure of this invention;

Fig. 6 is a cross-sectional view of a solenoid adjacent a pole piece;

Fig. 7 is a cross-sectional view of the compensating structure of this invention employed in connection with the structure of Fig. 6;

Fig. 8 is a cross-sectional view of a lilystron amplifier tube employing the structure of Fig. 7; and

Fig. 9 is a cross-sectional View of a traveling wave amplilier employing the compensa-ting structure of this invention.

with the existing solenoid structure and consists of para-1 u magnetic shells substantially concentric and coextensive with the solenoid elements. The shells are employed in-4 side the4 solenoid and surrounding the electron beam.u

The shells become saturated in the iields of the solenoid and set up their own magnetic iield, which when super` u imposed on the existing fringing magnetic field due to the gap of the solenoid effectively straightens the flux lines and results in a uniform axial magnetic iieldat the" the `desired configuration of Fig.` 3. yWhen the .ShelliA `structures `of Figs. 2 and 3 Referring now to Fig. l, there is shown in cross-section a segment of a long current carrying coil or solenoid 1. If the solenoid is uniformly wound the magnetic flux lines near the center of the length of the solenoid are substantially parallel to the axis of the solenoid, as shownin the gure. If now an electron beam be projected along this axis, the cross-sectional area 'of the beam will tend to remain constant due to the uniform focusing or if two solenoids having common axes are placed close together so that both windings produce aiding magnetic flux, a fringing magnetic ield will exist in the vicinity of the gap. The axially-directed iield near the axis will be reduced in the gap region. An electron beam traveling along the axis and being focused by the axial magnetic eld will tend to diverge or be defocused as it crosses the gap defining the decreased axial magnetic eld. The original cross-sectionalarea ofthe beam will not in general `be restored after crossing the gap.

p In order to overcome the defocusing effect on an axial beam of electrons by a gap in a solenoid the compensating structure of Fig. 3` is employed; The compensating structure consists of, a magnetized shellfhaving a gap 3 corresponding to the gap of the solenoid structure.

The shell may be composed of any paramagnetic material, such as cold-rolled steel. The portions 4 and 5 of the shell must be so magnetized that their respective ends 7 and 8 nearest the gap are unlike magnetic poles in order that fringing magnetic fields as shown are introduced at the gap. TheV fringing iields in the Vicinity of` the gap 3 of the magnetic shell are of such shape that they tend to compensate for the fringing fields in the vicinity of the gap 2 of the solenoid when the solenoid and shell are properly oriented with respect to each other. p

Fig. 4 shows the compensating magnetic shell portions 4 and 5 introduced in `the structure of Fig. 2 in` accordance with this invention. The shell portions 4 and 5 are concentrically mounted within respective solenoidl portions 9 and 10 arranged to be coextensive with these solenoid portions. Thepmagnetomotve force generated by the ,solenoid portions saturates the shell material` thereby inducing magnetic lields in the shell which have fields of Fig. 3 are superimposed on the fields due to the solenoid portions of Fig. 2, magnetic field compensation is achieved. In the area surrounded by lthe shell, the outwardly fringing linx lines of Vthe solenoid gap are straightened or corrected by the inwardly fringing flux lines of the shelf structure. As shown in Fig. 4, the flux lines are materially straightened in the vicinity of the gap, and an electron beam will remain focused as it traverses the gap of the solenoid along its axis.

The Value of axial magnetic field in the area of the path surrounded by the shell remains substantially the same as before, since the magnetomotive force is unchanged. The eect of the reduction in path reluctance on the magnetomotive force is overcome by the increased fiux in the shell. it `has been found that the number of linx lines inside the shell should be about twice the number of lines threading the area of the path surrounded by the shell. The saturated iron cylinder normally would have a flux density of about 137,000 lines per square inch. Equating the total flux within the cylinder to twice the liux threading the area surrounded by the shell, the following simplified relationship expresses the approximate design value for the thickness t of ythe shell to the radius R of the shell,

, in Fig. 5 in which there is shown in cross-section a three resonator liiystron amplifier tube employing a magnetically focused electron beam, the beam being focused-V The klyaccording to the principles of this invention. stron comprisesV an electron gun 2i for projecting a cylindrical stream of electrons through the successive toroidal cavity resonators 22, 23 and The beam is terminated in a coilector Z5. The resonant cavities 22, 23 and 3d are of the doubly reentrant type, their reentrant portions defining the respective interaction gaps 26, 27 and 28. VConnected to the cavity resonators for coupling'in or extracting energy therefrom are the coaxial line coupling elements 29, 3ft and 3i, which project radially from the respective cavity resonators to which they are connected. It is necessary for efficient operation of lthis tube that the diameter of the interaction gaps be kept as small as possible. In order to confine the electron beam to a sufficiently small cross-sectional area that it may pass through the three successive interaction gaps, it is necessary to supply a magnetic focusing field along 'the axis of the tube. The magnetomotive force to supply the field -is provided by the cylindrical 4solenoid sections 32 and 33. The solenoid Cannot be formed as one continuous cylindrical element-because of the radial coupling-elements,- -30 and Si. Paramagnetic pole' pieces 34 and 35 abutting the ends of the solenoid complete the magnetic path. In order to prevent fringing of 'the magnetic field at the central interaction gap 27, the

paramagnetic shell portions 36 andV 37 are provided sov as to be concentric and coextensive with thesolenoid portions 32 and 33. Theseshell portions 36 and 37 further act to correct the fringing magnetic field opposite the gaps defined by pole pieces 34; and 35 and respective solenoid sections 32 and v33, as will be explained later.

This invention is also useful for shaping or correcting the axial magnetic field near the ends of a solenoid.

Fig. V6 shows in cross-section the magnetic field existreaches an outletk aperture'43 in'pole piece 42. Instead A outputV connector 56 respectively introduce signal energy,k

. panying'drawings shall be interpreted as illustrative and r ergy therefrom.

of passing Ithrough the aperture 43, the `beam will strike the edges of the aperture and cause undue overheating and undesirable Secondary emission electrons.

The magnetic field near the end of the solenoid may be corrected by the compensating paramagnetic shell 44 of this invention, as shown in Fig. 7. The magnetic liux induced in the shell tends to bow or fringe inwardly near the axis of the solenoid at the gap 45 and thereby cornpensates or straightens the lines of iiux in the vicinity of the gap. These straightened lines of magnetic flux permit passage of a greater portion of the electron beam through aperture 43.

Referring once more to Fig. 5, the shell portions 36 and 37 tend to correct the axial focusing field in the respective gaps defined by the ends of solenoid sections 32 and 33 and pole pieces 34 and 35 in accordance with the principles described in connection with Figs. 6 and 7.

A modified version of the klystron amplifier of Fig. 5 is shown in Fig. 8, in which only one resonant cavity has a radially extending coupling element 3l. Thus the axial focusing solenoid 46 may extend for the full length of the tube with the exception of a gap in the vicinity of` coupling element 31. Consequently, a single saturable shell 47 may be inserted within the solenoid to provide the compensation required at interaction gap 28 for passage of 'the electron beam.

A further embodiment of this invention is shown in Fig. 9, wherein the electron beam of a traveling wave amplifier is focused. The traveling wave amplifier essentially comprises an electron gun 51, a slow wave structure 52, such as a helix, a collector 53 and a vacuum envelope 54. An R.-F. input conector 55 and' an R.-F

onto vthe slow wave structure and extract amplified enof electrons along the axis of the slow wave structure, where the electron beam interactsV with the electromagnetic wave traveling on the structure and causes amplification of the wave. The beam then passes through an aperture 57 to collector 53. The electron beam is focused throughout its travel along the tube axis by means of an axial magnetic field provided by a solenoid 58 in cooperation with paramagnetic pole pieces 59 and 60. The R.F. output connector 56 which projects radially from the vacuum envelope prevents the solenoid 58 from abutting pole piece 60, the resulting gap between the end of solenoid 58 and pole piece 60 causing a fringing or reduced axial magnetic eld in the vicinity of aperture 57. VCompensation lor correction of the .fringing field in the vicinity Vof, the aperture is provided by the paramagnetic shell or insert 61, which is coextensiveV with solenoid 58 along theV axis of the tube. Asudiscussed in'connecton with Figs. 6 and 7 thisma'gnetic insert corrects'the axial flux lines in the vicinity of the magnetic gap thereby permitting a greater portion of the electron beam to pass through the aperture 57 to the collector 53.

Since many changes could be made in the'aboveconstruction and many apparentlyV widely different embodiments of this inventioncould be made without departing from the scope thereof, it is intended that all matter contained in the above description or shownrin the accomnot'in a limiting sense.

What is claimed is:

i l. The combination of means for producing anddirecting an'electron stream along a predetermined axis, first i and second apertured solenoid sections disposed in coaxial relationship with said axis forproducing an axial magnetic field that confines saidV stream to a giyen Vcross section, the'adjacent ends of said solenoid sections being la'tionsliipA 'with its surrounding solenoid section and sepa- The electron gun 51 projects a stream where B is the axial ux density in gauss of the flux lines threading the area surrounded by the shell.

2. The combination of means for producing and directing an electron stream along a predetermined axis, at least one apertured solenoid section and at least one apertured pole piece disposed in coaxial relationship with said axis for producing a magnetic field that confines said stream to a given cross section, one end of said solenoid section being separated from said pole piece by an air gap, a thin-walled shell positioned coaxially within the aperture of said solenoid section in substantially coextensive relationship therewith, the shell being separated from said pole piece by an air gap, said shell being of a magnetically permeable material that is `saturated by the magnetic field which is produced for coniining said stream to its given cross section, the saturated flux density of said shell being of the order of 137,000 lines per square inch, the thickness t and radius R of said shell having an approximate relationship of -5 R- 4.71 X 10 B where B is the axial flux density in gauss of the ux lines threading the area surrounded by the shell.

References Cited in the file of this patent UNITED STATES PATENTS 2,225,447 Haeff et al Dec. 17, 1940 2,259,531 Miller et al Oct. 21, 1941 2,305,884 Litton Dec. 22, 1942 2,608,668 Hines Aug. 26, 1952 2,664,514 Reiches et al Dec. 29, 1953 2,681,421 Gethmann June 15, 1954 2,707,758 Wang May 3, 1955 2,733,364 Flory Jan. 21, 1956 2,741,718 Wang Apr. 10, 1956 2,807,743 Ciolii Sept. 24, 1957 2,822,500 Bryant Feb. 4, 1958 2,829,299 Beck Apr. 1, 1958 2,841,739 Pierce July 1, 1958 2,843,788 Peter July 15, 1958 2,843,789 Klein et al. July 15, 1958 

